The presented invention relates to a battery, especially a flat cell, comprising a first electrode of lithium-metal or lithium-alloy, a second electrode of an active material intercalating lithium-ions, a separator between both electrodes and a housing with electrical connections enclosing the electrodes and the separator.
The prevailing problem with this type of batteries is to efficiently discharge or recharge the electrode with the active material due to the limited quantity of the active material available. In order to improve the electrical conductivity of the electrode it was suggested (U.S. Pat. No. 5,470,357) to laminate this electrode with an electrical connection. This solution, however, only results in a gradual improvement of battery performance. With the existing batteries of this kind one also has a conflicting situation between the current capacity and the run-time of the cell insofar as an increase of run-time necessitates an increase of electrode mass. Conversely, an increased electrode mass has a negative effect on ion-mmigration (longer distances) and therefore on internal resistance of the electrode.
U.S. Pat. No. 5,219,673 describes a battery where the battery cell is formed by a coiled multi-layer body. Here also one has the general conflict that the electrodes cannot have a desired bigger mass as the ion-migration within this mass would lead to an increase of the internal resistance.
U.S. Pat. No. 4,830,940 describes a battery where the anode is built as an elongated strip folded multiple times in order to accommodate cathode plates within the separate folds. Thus a compact cell is built and lodged in a common housing. The cathodes and the wound around anode are provided with connector-tabs at different locations. The multi-layer design increases the internal resistance of the cell, however.
The object of the present invention is to present a battery of the type described above in which
electrical conductivity and recharging are considerably improved,
internal resistance of the electrodes is optimized,
current capacity is increased and
run-time is extended.
According to the present invention, these objectives are reached by the fact that at least one of the electrodes (1; 2; 3) is a multi-layer body built by multiple folds and by an equal layer-thickness of the active material (4; 12; 13) between the folded layers. This way the electrode carrier material permeates the active material multiple times. This results in an optimized internal resistance within the active material layers (improved ion-migration) comprised within the different folds. In the same time, current capacity of the cell built according to the invention will be improved due to a significant increase of the electrode surface area. Furthermore, the run-time of the cell is considerably increased because of the much bigger amount of active material available within the electrode.
According to a preferred embodiment of the invention the folding pattern is a Leporello (zig-zag)-fold. In this context it is advantageous, according to the invention, that a carrier material (5) permeable for ions is coated with active material (4) of equal layer-thickness on both sides.
This way the overall thickness of active material between the folds of the Leporello-fold always is equal. This, in turn, ensures homogeneous current distribution. Within the scope of the invention, it is also possible that the Leporello is coated on both sides with layers of active material of different coating thickness. In this case it is advantageous that the Leporello is coated on one of its sides with a thin layer of active material and on its other side with a thick layer of active material (12) on every other fold. A thickness-ratio of 1:2 will result again in equal overall thickness of active material between the folds of the Leporello.